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Biological Control of Insect Pests
on Field Crops in Kansas
J.P. Michaud, P.E. Sloderbeck, and J.R. Nechols
beneficial practices can be integrated into production
systems in a cost-effective and convenient manner. This
requires not only a good understanding of the biology
and ecology of natural enemies, but also a willingness to
modify production practices to accommodate their needs.
Complicating matters is the fact that many beneficial
insects feed not only on pest insects, but also on each
other, a phenomenon referred to as intraguild predation.
In a healthy agroecosystem with a high degree of plant
and insect diversity, naturally-occurring biological control
can go almost unnoticed because natural enemies are
effectively maintaining a plethora of potential pests at low
densities.
The most obvious harmful practice is the use of
insecticides at times when natural enemies are active.
Insecticides have a wide range of adverse effects on
natural enemies, killing them directly, impairing their
foraging and reproductive abilities, and depriving them of
food. Nevertheless, there are various ways that insecticides
can be successfully integrated into a production system
while minimizing their impact on beneficial species.
Materials such as certain varieties of Bacillus
thuringensis (Bt) are selectively toxic to particular groups
of pest insects such as caterpillars and leave natural
enemies unharmed. Unfortunately, most Bt formulations
currently available lack good residual efficacy under
field conditions. The in-plant expression of Bt toxin in
genetically-engineered corn and cotton varieties has
revolutionized the management of moth pests on these
crops and has so far proven compatible with biological
control of other pests, primarily through reduced
applications of hard insecticides.
Several recently developed biorational materials such
as spinosad and indoxacarb achieve selectivity through
low contact toxicity; they must be consumed by the
insect to be activated. Since natural enemies typically
don’t eat plant material with the chemical on it, they are
spared direct mortality. However, even materials with
high contact toxicity can be applied in ways that minimize
their impact on beneficial species. Insecticides with good
plant systemic activity (e.g., clothianidin, imidacloprid,
thiamethoxam) can be used as seed treatments, in-furrow,
Biological Control
In the simplest terms, biological control is the
reduction of pest populations brought about through
the actions of other living organisms, often collectively
referred to as natural enemies or beneficial species.
Virtually all insect and mite pests have some natural
enemies, although not all are effective in suppressing pest
populations. Learning to recognize, manage, and conserve
natural enemies can help reduce pest populations and
maintain them below economic levels, thus reducing crop
losses and the need for more costly control measures that
may also have undesirable environmental side-effects.
Biological control is often most effective when
coupled with other pest control tactics in an integrated
pest management (IPM) program. Practices that are
often compatible with biological control include cultural
controls, crop rotation, planting pest-resistant varieties,
using insecticides with selective modes of action, or spot
treatments that leave untreated areas to serve as refuges
for natural enemies.
Effective biological control often requires a good
understanding of the biology of the pest and its natural
enemies, as well as the ability to identify various life
stages of relevant insects in the field. Field scouting
usually is necessary to monitor natural enemy activity,
evaluate impact on pest populations, and anticipate the
need for additional control measures. Although three
distinct approaches to biological control are recognized
(conservation, importation, and augmentation), the
principles of conservation biological control are by far the
most important for producers of field crops in Kansas to
understand.
Conservation of Natural Enemies
Natural enemy conservation is at once the most
straightforward concept of biological control in the
context of field crops, and the most complex. At the
simplest level, conservation biological control means
avoiding cultural practices that harm natural enemies
and implementing practices that attract, encourage, or
benefit them. Although this may seem like common
sense, the challenging part is understanding exactly
what practices are harmful to natural enemies and how
1
or as soil drench applications to be taken up by the
plants while causing little direct mortality to natural
enemies. Finally, damaging pest populations are often
confined to portions of a field, rather than being evenly
distributed throughout it. Selective spot-treatment of
affected areas will not only reduce application costs, it
will leave untreated areas to serve as reservoirs for natural
enemies. These survivors can then recolonize treated areas
following degradation of the insecticide, accelerating
the restoration of biological control and sometimes
averting the need for subsequent treatments. Nevertheless,
situations arise where the preservation of biological
control requires avoiding the broadcast application of any
insecticide. When biological control is widely disrupted
by insecticide applications, minor pests can become major
pests (secondary pest resurgence), and a farm can develop
dependency on chemical control measures once natural
enemies are no longer resident in the fields (the pesticide
treadmill effect).
Certain cultural practices also can be detrimental
to natural enemies. Plowing, cultivation, mowing, or
harvesting operations can be disruptive to natural enemies
if they coincide with critical stages of their life cycle.
While the adoption of no-till and minimum tillage
agriculture has favored the resurgence of some pests such
as false wireworms that utilize crop residues for food or
harborage, such practices also favor various beneficial
insects such as ground beetles, spiders, and other
generalist predators that rely on crop residues for cover,
which encourages their persistence in the field. Dust
raised by traffic along dirt roads or cultivation operations
carried out during dry weather can impede the foraging
activities of beneficial insects when it is deposited in
sufficient amounts on leaf surfaces. The burning of crop
residues can also kill large numbers of beneficial insects, as
can inappropriately timed flood irrigation. Other practices
such as excessive herbicide applications on pastures and
fallow fields reduce both plant and insect diversity in noncrop habitats that normally serve as important reservoirs
of many natural enemy species. These beneficial species
are then no longer available to colonize annual crops in
adjacent fields as pest populations develop.
One of the biggest hurdles for sustainable biological
control in field crops is the loss of plant and insect
diversity associated with large scale monoculture,
a configuration that tends to favor pests over their
natural enemies. Conservation of natural enemies can
be generally improved by avoiding a completely clean
farming approach and by taking steps to encourage plant
and insect diversity in non-cultivated areas and wherever
else this may be feasible. Strip-harvesting of alfalfa,
where alternating rows of alfalfa are left uncut until the
cut areas begin to grow back, is an excellent example of
conservation biological control because this modified
harvesting practice provides a continuous refuge and food
supply for beneficial insects.
Importation of Natural Enemies
Today’s high volumes of international trade and
airline traffic have increased the frequency of arrival
of exotic pests from other regions of the world. When
immigrant pests succeed in establishing in a new
geographic location they can rapidly reach very high
populations and cause serious economic losses, largely
because they lack the complex of natural enemies that
limits their population growth in their country of origin.
Examples of serious pests in Kansas of foreign origin
include the Hessian fly, the European corn borer, the
Russian wheat aphid, and the alfalfa weevil. The selective
importation and release of natural enemies from the pest’s
country of origin is also known as classical biological
control. This approach gained impetus early in the last
century following several dramatic successes, notably
the importation of the vedalia beetle to control cottony
cushion scale in California. However, only a small
proportion of importations have met with this level of
success and the general applicability of this approach to
all new invasive pests is currently a subject of considerable
debate. Although large populations are often observed
when an exotic pest first invades, these often decline
over a period of years as the native community of natural
enemies gradually evolves to exploit them. Evidence is
growing that many invasive pests ultimately come under
adequate biological control solely through the action
of native beneficial species. However, there are other
examples where native natural enemies do not — or
cannot — provide required levels of biological control.
Classical biological control involves exploring a
pest’s country of origin for a potentially effective natural
enemy, importing it to the pest’s adopted country,
and mass-rearing it in the laboratory for subsequent
release in regions where the pest is active. The goal is
to ultimately establish a self-sustaining population of
the natural enemy that maintains the pest population
below economic threshold levels for perpetuity. In this
regard, classical biological control differs from other
forms of biological control in that it is not carried out
by the farmer or gardener, but only by scientists with
appropriate authorization from federal agencies, in
particular the United States Department of Agriculture.
Non-native insects must be held under strict quarantine
conditions until it can be ascertained that (1) they have
some potential to control the target pest, (2) they will
not themselves become pests, and (3) they do not harbor
their own natural enemies that might interfere with
their effectiveness or that of other beneficial insects. In
addition, prospective natural enemies are evaluated to
determine their potential to attack and/or feed on other
beneficial species. Growing awareness and concern about
potential non-target effects of released natural enemies
has led to increasingly stringent criteria for introductions.
These restrictions are justified given that other, far
more complex, ecological impacts of introduced species
are exceedingly difficult to predict. A good example is
the multicolored Asian lady beetle, Harmonia axyridis.
Although this species is an excellent biological control
agent and a voracious predator of aphid species on many
crops, it appears to have displaced many native lady
beetles from their habitats and become a serious urban
pest in many regions.
Augmentation of Natural Enemies
To some people, biological control means buying and
releasing natural enemies to control pests. This approach
is generally known as augmentation, although in the
strictest sense, augmentation refers to situations whereby
natural enemies are released to supplement an existing
population, something that is rarely done in field crops.
Augmentative biological control is widely recognized by
the public because many commercially available insects
are advertised for sale in magazines and publicized in
the media. Further, the use of pesticides has conditioned
us to think about pest management in the context of
purchased products. However, in contrast to the other
two forms of biological control, augmentation is less
sustainable because it relies on regular or periodic releases
of purchased products, something that sometimes benefits
producers of these products more than consumers.
Situations do exist where augmentation can be highly
efficacious, cost effective, and a desirable alternative to
pesticide applications, but these occur mostly in enclosed
settings such as greenhouses and interior plantscapes.
Natural enemy augmentation is based on the
assumption that, in some situations, there are not
adequate numbers of natural enemies to provide sufficient
biological control (even though some may be present),
or that their immigration is not timely enough to
suppress the pest before it reaches economic levels. One
requirement for augmentation to be feasible is a source
of large numbers of natural enemies that are readily
available for an affordable price. In response to demand,
many companies have developed insectaries capable
of producing large numbers of predatory and parasitic
insects and others produce and market nematodes and
microbial pathogens as other forms of biological insect
control. Unfortunately, while these companies may have
developed good techniques for rearing and disseminating
their beneficial insects, they typically have limited
experience with regard to how, where and when to release
these insects into the crop for greatest impact. Likewise,
only rarely do recommendations identify circumstances
when releases of natural enemies should not be made.
There are two general release strategies in
augmentative biological control: inundative and
inoculative. Inundation involves releasing large numbers
of natural enemies for immediate reduction of a damaging
or near-damaging pest population. This strategy is used
mainly for short-term control and is only feasible for
natural enemies that can be produced in large numbers
very cheaply. It is applied as an immediate, corrective
measure; successful reproduction and continued survival
of the natural enemy population is neither presumed nor
expected. Inundative releases are often used as a substitute
for a chemical spray that might be undesirable because of
unwanted side-effects, hence the term biopesticide. In
contrast, inoculation involves releasing small numbers of
natural enemies once or more throughout the period of
pest activity, usually beginning when the pest is still at low
density. It is therefore more of a preventative measure, in
that the natural enemy is expected to reproduce and build
a population that will prevent the pest from reaching the
economic injury level.
Augmentative biological control is only reliably
effective when it rests on a solid foundation of research
derived from the specific context of application. Although
many purveyors of natural enemies recognize the need
to provide an insect + information package to their
clientele to maximize the chances for success, many others
are primarily concerned with selling insects, with the
result that many are sold for inappropriate applications,
sometimes generating consumer dissatisfaction with
biological control in general. A prime example is the
sale of adults of the lady beetle, Hippodamia convergens,
to control aphids. Dozens of outlets offer these insects
for sale, including some of the largest agricultural
supply companies, despite general agreement among
entomologists that they are not effective in this
application. Most commonly, the adult beetles are
collected in buckets en masse from overwintering
aggregations in the mountains of California. This practice
is fraught by at least two problems. As they emerge from
hibernation, the beetles instinctively disperse rather than
feed and lay eggs. Consequently, most fly away from
their release sites immediately, regardless of the presence
of aphids. Secondly, beetles collected from hibernation
are typically in reproductive diapause, a dormant state in
which eggs are not laid and adult feeding may be minimal,
resulting in poor performance. A general problem, not
specific to lady beetles, is that populations reared in the
insectary, or field-collected from certain localities, may be
adapted to specific environmental conditions substantially
different from those where they are released. If local
climate or food sources are considerably different from
those to which the populations is adapted, survival and
reproduction may be greatly diminished at release sites.
Even when an appropriate natural enemy is selected,
failures to achieve satisfactory control can result from
a number of causes, most commonly a user’s lack of
information on the biological requirements of the insect,
the biology of the target pest, and effective modes of
application. Another problem is that appropriate release
rates are difficult to determine for a given situation and
depend on many factors, not least of which is the actual
pest density at the time of release. The best advice for pest
managers considering an augmentation approach is to
do their own research and obtain as much information
as possible to maximize the probability of success. Wellresearched projects can and do result in very effective
augmentative biological control programs, including those
involving applications of microbial insecticides.
The cost of commercially supplied natural enemies
is a major consideration in assessing their potential
suitability as an alternative for pesticides in any situation.
Prices vary widely because of differences in the degree
of difficulty and expense in rearing different species.
Sometimes it is justifiable to pay a higher cost for natural
enemies relative to an insecticide, provided adequate
control of the pest is obtained. This may be the case
where pests have developed insecticide resistance, where
worker protection standards are a concern, where there are
risks of disrupting biological control programs for other
pests with insecticides, or where a premium price can
be obtained for certified organic production. In general,
the inoculative approach is more cost effective provided
that correct release rates and timing are used. Inundative
releases can be justified on high-value crops where the
cost of production is already substantial. Managers should
carefully evaluate the cost of a natural enemy as they
would any other production cost before making a decision
on augmentative release.
It can provide a safe alternative for controlling certain
pests in some situations, but a significant investment in
research is required for it to provide reliable control in any
given situation. It is the responsibility of the end user to
obtain and assimilate the relevant information necessary
for effective implementation of a release program and
ensure that the product purchased is appropriate for the
particular pest and situation. There are no commercially
available natural enemies that are currently recommended
for augmentative biological control applications in largescale commercial production of field crops in Kansas.
Recognition of Common Beneficial Insects in Kansas
Field Crops
Predators
By predators we refer to insects or other arthropods
(spiders and mites) that feed on other insects (prey) by
hunting, killing and directly consuming them. Although
more than 100 families of insects contain predaceous
species, about 12 of these contain the major biological
control agents of field crop pests. Here we summarize the
biology and description of some of the most important
families.
Lady Beetles (Coleoptera: Coccinellidae)
Lady beetles, or ladybugs, are possibly the most
universally recognized group of beneficial insects. They
are found almost anywhere and usually feed on aphids
and a variety of other soft-bodied insects. Some will
also feed on mites and the eggs and larvae of moths and
beetles. Currently there are about five species that can be
commonly encountered in field crops in Kansas.
Probably the most abundant species in Kansas field
crops is the convergent lady beetle, Hippodamia convergens,
a native species. This lady beetle is recognizable by the two
convergent white lines on the black portion of the body
immediately behind the head (Fig.1). Coloration ranges
from pale orange to red with a series of black spots that
may only be faintly visible, or entirely absent. Aphids are
the preferred food, although adults may supplement their
diet with other prey items and certain vegetable food
Important questions to ask before considering an
augmentation program:
1. Has research shown that a release program can be
effective for a particular pest, crop and local situation?
2. What is the best time to release the natural enemy in
relation to the pest’s life cycle?
3. Are releases compatible with other crop production
practices that are anticipated, including the possible
need to apply pesticides against other pests?
4. Does the supplier provide a comprehensive information
package with clear instructions on handling, releasing,
and evaluating the effectiveness of the natural enemy?
5. What quality control practices does the supplier use to
ensure that insects arrive alive and in good condition?
6. How does the overall cost of a release program compare
with alternative control strategies when all ancillary
costs and benefits are factored in?
To summarize, augmentation is perhaps the most
publicly recognized form of biological control, but also
one of the least understood and most often misapplied.
Fig. 1
sources. Overwintered adults produce one generation in
early spring that is typically completed around the time
wheat is harvested, leading to the mass exodus of large
numbers of beetles from maturing wheat fields. Firstgeneration females mate within days of emergence, but
produce very few mature eggs right away unless they are
able to encounter a large supply of aphids. Rather, the
majority produce fat bodies to store the energy gleaned
from whatever prey they find and forgo reproduction
for the duration of summer, surviving dry periods by
drinking the sap from various plant species, especially
sunflower. As fall brings cooler weather, aphids once
again increase in abundance and the over-summered
females lay their bright yellow or orange eggs in clusters
near aphid colonies to begin the next generation. The
number of generations is variable, depending on the food
supply, but the majority of adults maturing late in the
season delay reproduction and conserve their resources for
hibernation. They crawl into protected sites, typically at
the base of grass tussocks, and remain dormant through
winter months until they are awakened by the warm
temperatures of spring. These abilities to hibernate in
winter and forgo reproduction during summer when prey
are scarce represent adaptations that are likely key to this
species’ success and abundance in prairie habitats.
Another common species is the twelve-spotted or
pink lady beetle, Coleomegilla maculata. The adults are pink
with six black spots on each wing cover (Fig. 2). Aphids
are also the preferred food of this species, but a very wide
range of insect prey may be consumed. The larvae of this
species are unique among lady beetles in their ability to
complete development on an exclusive diet of pollen.
corn borers. This species is often abundant in proximity
to water sources such as rivers and lakes. It hibernates
in aggregations during winter months but, unlike the
convergent lady beetle, probably continues to reproduce
throughout summer months, typically producing 3 or 4
generations per year.
A species becoming increasingly common in
the Midwest with the advent of soybean aphid is the
multicolored Asian lady beetle, Harmonia axyridis (Fig.
3). Adults are highly variable in coloration (pale orange
to red) and spotting patterns (males often have none).
The key to identifying this species is the prominent
black ‘W’ on the part of the body behind the head. This
Fig. 3
invasive species is a voracious predator of aphids and
many other insects, including the larvae of alfalfa weevil.
Unfortunately, it can also complete development feeding
on the eggs and larvae of many other lady beetles and has
been implicated in the declining abundance of a number
of native species, a fact that has marred its reputation
as an otherwise effective biological control agent in
many types of agricultural production. Another habit
contributing to the potential pest status of this beetle is
a propensity for entering houses in fall and winter, often
forming large aggregations that can be very distressing to
homeowners.
The seven spotted lady beetle, Coccinella
septempunctata, is another imported species, one that
originates in Europe. It is a large beetle that can be
recognized by the distinctive seventh black spot that spans
the front edge of both wing covers and is flanked by two
small white triangles (Fig. 4). It prefers to feed on aphids
infesting grasses and herbaceous plants.
Fig. 2
Pollen is also attractive to adults and many can be found
in corn fields at tasseling, although they also like to feed
on the eggs and small larvae of many moth species that
are abundant in corn, including economically important
Fig.6
Fig. 4
Other, smaller species of ladybeetles can also be
abundant in Kansas fields, but may go unnoticed because
of their small size and more secretive habits. Many are
important for feeding on the eggs of moth pests such
as the European corn borer. Scymnus spp. (Fig. 5) have
larvae that produce waxy secretions that serve to defend
them against ants, causing them to resemble mealy bugs
Fig. 7
Fig.5
(Fig. 6). Others such as Stethorus spp. are even smaller and
specialize in feeding on mites (Fig. 7).
Most aphid-feeding species lay bright yellow or
orange eggs in clusters of 15 to 40, usually near, but not
directly in, aphid colonies. The larvae of ladybeetles are
more difficult to identify to species than adults, but larvae
of most of the larger species resemble little alligators.
There are four larval instars (molts) before the pupal stage
is reached.
Fig. 8
Hover Flies (Diptera: Syrphidae)
Hover flies (Fig. 8) are also known as flower flies and
are most easily recognized by their hovering flight above
flowers or aphid-infested plants. Many resemble bees
or wasps, but different species vary greatly in size and
appearance. The larvae (Fig. 9) are voracious predators of
aphids and, in some cases, other small, soft-bodied insects.
by the larva in its period of development. The pupae are
typically teardrop-shaped (Fig. 11) and may form on plant
parts below the aphid colony, or in the soil, depending on
Fig. 9
Fig. 11
Adult hover flies require access to flowers as they
need sugar from nectar to fuel their flight. In addition,
female flies must consume pollen as a source of protein
before they can mature their eggs. Adult females find
aphid colonies by orienting to the smell of honeydew
excreted by the aphids. Many studies have shown that
planting mixed borders of wildflowers around gardens
can attract hover flies and increase the rate of egg-laying
on aphid colonies on adjacent vegetables and other crops.
This is an example of conservation biological control by
habitat management and is the only effective means of
encouraging these insects.
Different species of hover fly vary in the kinds of
aphid colonies they select for laying their eggs and many
are quite specific to certain aphid species. The white,
oblong eggs (Fig. 10) are usually laid singly in among the
species. The pupa is also the overwintering stage. Syrphid
flies comprise an important, if often overlooked, source
of aphid mortality that, acting in concert with other
predators and parasitoids, is important in keeping aphid
populations below damaging levels.
Lacewings (Neuroptera: Chrysopidae and
Hemerobiidae)
All lacewings are predaceous as larvae, and adults of
some species are predaceous as well. Lacewing larvae (Fig.
12) prefer aphids as prey but also consume a range of
other soft-bodied pests such as mites, thrips, leafhoppers
and mealybugs. The most common species in Kansas field
Fig. 10
Fig. 12
aphids. A maggot hatches from the egg in 2 or 3 days and
begins to feed on aphids voraciously, growing at a truly
remarkable rate. The larvae are slug-like, tapered toward
the head, and use a film of their own saliva to adhere
to the leaf surface. There are typically three instars that
may be completed within 7 to 14 days, depending on
temperature, and as many as 400 aphids may be consumed
crops is the green lacewing, Chrysoperla carnea, although
other species also occur. Adults have large, lacey wings,
thread-like antennae, and protruding eyes (Fig. 13). They
are primarily nocturnal, but when disturbed, they will
leave their resting places on the undersides of leaves in an
erratic, fluttering flight. Despite their fragile appearance,
lacewings are among the very few insects capable of
extricating themselves from a spider’s web. The white,
Fig. 15
contents like soup through a straw. This process is known
as extraoral digestion (which is also used by lacewing
larvae). A wide range of prey are taken depending on the
size and species of bug. Minute pirate bugs are partial to
thrips, but other bugs will feed on aphids, caterpillars, and
insect eggs. The larger assassin bugs (Reduviidae) are top
predators in the food chain and feed on many kinds of
insects, even lady beetles. This is an example of intraguild
predation – predators eating each other as well as feeding
on pest species, a phenomenon that often adds complexity
to biological control systems.
Fig. 13
Ground Beetles (Coleoptera: Carabidae)
Ground beetles are commonly found in all cultivated
field crops. Both larvae and adults are predaceous on many
ground-dwelling insects, but their actual contribution to
biological control of crop pests is not well understood.
Most species are large, shiny and black, with ridged wing
covers. They have threadlike antennae and a head that is
smaller than their thorax. Adults of most species rarely fly
and are most often seen running across the soil surface.
Most adult feeding occurs on the soil surface, and most
larval feeding under the soil surface, so they feed on root
maggots, rootworms, caterpillars, and other soft-bodied
insects that might be dislodged from plants. For example,
ground beetles have been studied for their contribution
to biological control of cereal aphids, even though most
will not climb a plant to reach the aphids. However, as
lady beetles and other predators attack an aphid colony,
they often dislodge more aphids than they actually eat,
and carabid beetles foraging on the soil surface reap the
rewards, and ensure that these aphids are not able to
climb back onto the plants.
Fig. 14
oval eggs are laid on the end of long stalks (Fig. 14). The
relative length of the stalk and the pattern in which the
eggs are laid (singly versus in groups, in line or in a spiral)
can be characteristic of particular species.
Lacewings are among the beneficial insects that are
available commercially. Usually, the eggs are shipped
mixed with a substrate such as rice hulls and some moth
eggs for food. However, because the larvae are highly
cannibalistic and immediately begin to kill and eat each
other upon hatching, they require immediate distribution
in the field. Even then, lack of biological information on
habitat preferences, climatic tolerances, dormancy, and
the behavioral responses of particular species, as well
appropriate release rates and suitable release techniques,
have severely limited the usefulness of these insects in
augmentative biological control programs.
True Bugs (Hemiptera: Anthocoridae, Nabidae)
Numerous species of ‘true’ bugs are predators of insect
pests. These include damsel bugs, (Nabis spp., Fig. 15), bigeyed bugs (Geocoris spp.), minute pirate bugs (Orius spp.)
and assassin bugs. Predatory bugs skewer their prey with
piercing and sucking mouthparts, inject enzymes to digest
the internal organs, and then drink the liquefied body
Spiders
Although spiders are not insects, they play an
important role as generalist predators of many insect
groups. As such, their role as biological control agents is
perhaps greater than previously thought. Their presence
also is considered by many to be indicative of a healthy
agroecosystem. Spiders comprise a very diverse group that
can be broadly categorized by their hunting strategies.
Web-spinners, including orb-weavers, and garden spiders,
use silk to trap their prey in various ways. Species such as
crab spiders are highly cryptic ‘sit and wait’ predators that
hide in flowers to ambush pollinators. Hunting spiders are
typically hairy, robust species that do not build webs, but
actively seek out their prey. These include jumping spiders,
wolf spiders and the large tarantulas.
controlling the fertilization of eggs; males are produced
from unfertilized eggs and females from fertilized
ones. In some species, all-female lines persist for many
generations without sexual reproduction. The female uses
an ovipositor to lay eggs in a host insect (the stinger of
a honey bee is a modified ovipositor that delivers only
venom). In some species the ovipositor is held internally
when not in use; in others it is not retractable and may
be as long, or longer than, the entire wasp body. Venom
that serves to immobilize, paralyze or otherwise subdue
the host may also be delivered via the ovipositor. Some
female parasitoids also use the ovipositor to puncture a
host and then feed on the body fluids before selecting
other hosts for oviposition, thus causing two different
types of mortality in the pest population. In some cases,
the egg is laid externally on the body of the host and the
larvae may also feed externally (ectoparasitism). More
commonly, the larva develops and pupates within the host
body, feeding selectively on the host’s internal tissues and
leaving the digestive tract and nervous system to the very
last (endoparasitism). Another important distinction is
whether the host is allowed to develop and grow with
the parasitoid larva inside it, or whether it is killed or
permanently paralyzed by the attacking females so that it
remains a static, rather than dynamic, food source for the
developing larva.
Parasitoids
Parasitoids are like the vampires of the insect world.
As immatures, they obtain their nutrition by feeding in
or on the body of another insect, ultimately killing it.
The adults are typically free-living and the females are
responsible for finding host insects for their progeny. The
two major groups discussed here are parasitic wasps and
tachinid flies.
Parasitic Wasps
Parasitoid wasps comprise one of the most diverse
and important groups of beneficial insects. Almost all
insects are attacked by at least one species of parasitoid,
and most by more than one. Some species attack only
one insect host and many successful classical biological
control programs have involved the introduction of highly
specific parasitoids. Many species are large and colorful,
but most of the economically important ones are small
and very inconspicuous. For example, those attacking
aphids are smaller than their aphid hosts (Fig. 16), and
those developing within a single moth egg or scale insect
Tachinid Flies
This group represents a very large family of flies
with more than 1,000 species in North America, all of
which have a parasitic lifestyle. They vary considerably in
appearance, but most have bristled bodies and resemble
house flies, although they can be substantially larger or
smaller (Fig. 17). The adult female typically lays an egg
on the surface of the host insect cuticle, and the hatching
Fig. 16
Fig. 17
are barely visible to the naked eye. Some are solitary, with
only a single individual completing development in a
host insect, whereas others are gregarious, with as many
as several hundred siblings feeding and developing on
the same host. This condition most often results from the
female laying many eggs on the same host, but in some
species a single egg undergoes a series of divisions before
development begins, a condition known as polyembryony.
Parasitoid biology is distinct from that of many
other insects. Although reproduction is typically sexual,
most females can manipulate the sex of their progeny by
larva then bores into the body of the host and develops
internally. In other cases, the fly egg is consumed by the
host insect while it feeds. Some species give birth to live
larvae that are placed directly onto the host. A wide range
of moth and butterfly larvae are attacked, and so are a
number of beetle species. The host may be killed in the
adult stage, but more commonly in the pupal stage.
Conclusions
Biological control is a natural process that plays an
important role in the suppression of field crop pests in
Kansas. A lot of the best examples of biological control
proceed completely unnoticed by the farmer, simply
because both the pest and its natural enemies coexist at
such low densities that there is no perceptible problem
in the crop, even thought the pest is present. This fact
has been demonstrated by using pesticide treatments to
disrupt natural enemy populations and then observing
formerly insignificant insect populations rise to the status
of major pests. It is generally agreed that integrated
pest management (IPM) is the preferred approach to
sustainable pest control in agriculture. Whenever possible,
IPM programs for field crops should be constructed on a
foundation of biological control, with additional control
measures applied only as needed and selected to conserve
natural enemies and all non-target insects to the greatest
extent possible. The best way a farmer can improve his
chances of benefiting from biological control (e.g. saving
the expense and hazard of pesticide applications) is by
learning to recognize those beneficial insects that are
important natural enemies of the key pests attacking his
crops. These insects should be thought of as farm laborers
that work for free — the only wages they demand are the
pests that they consume. While more research is needed
to better understand the needs and requirements of many
beneficial species so that their effectiveness might be
enhanced, there are many recognized techniques available
to generally conserve natural enemies and encourage their
activities. The advent of no-till agriculture has likely had a
net positive effect on the abundance of beneficial insects
in field crops. Crops genetically modified to express
natural insect toxins have, to date, proved either neutral
or favorable to biological control by virtue of reducing
overall pesticide usage. Fallow areas around fields can
serve as reservoirs of many natural enemies, especially if
weed species are allowed to flower. If a pesticide treatment
becomes necessary, leaving less-affected portions of a field
untreated can provide a refuge for natural enemies and
accelerate the field-wide restoration of biological control
post-treatment. Some newer pesticide formulations are
designed to be more selective for particular pests and
will spare natural enemies. In short, whenever biological
control has a role in pest population reduction, pest
control decisions should be weighted by considerations of
how natural enemies will be impacted, and what tactics
Nematodes
Nematodes are a phylum of roundworms that are
among the most abundant multicellular organisms on
earth. Although many families of nematodes feed on
plants and include many important pest species, some
are free-living whereas others are obligate parasitoids of
insects and include many important biological control
agents. Some are produced and sold commercially for
control of soil and foliar insects. Nematodes are normally
applied as either a spray suspension or a soil drench, but
their survival and efficacy is often dependent on soil type
and adequate moisture. They are associated with a number
of different commensal and symbiotic bacteria that aid
them in killing and digesting their host insects.
Microbial Pathogens
A variety of microbial pathogens, including bacteria,
protozoans, viruses and fungi are specifically pathogenic
to insects and completely harmless to other forms of
life. This selective pathogenicity renders many of them
valuable as biological control agents of insect pests.
Some, such as Bacillus thuringensis, have been the source
of natural insect toxins that are now synthesized as
biopesticides, or engineered directly into crop plants.
Many insect diseases caused by pathogens are very
important sources of mortality in pest populations
and lead to precipitous population declines when
they become epizootic (analogous to an epidemic in a
human population). However, many attempts to induce
epizootics in pest populations by means of distributing
spores or other types of inoculum fail because stringent
environmental conditions are often necessary for
successful infection and/or transmission of the disease.
For example, many fungal diseases of insects require
high humidity or prolonged leaf wetness in combination
with particular temperatures to infect their hosts.
Some success has been obtained with baculoviruses
commercially formulated with sunscreens to protect them
from solar radiation when sprayed onto plant surfaces.
However, most insect epizootics proceed without human
assistance when suitable environmental conditions arise.
Conservation is also a consideration when epizootics are
a significant natural mortality factor in pest populations.
For example, excessive use of fungicides to control
powdery mildew and other foliar diseases in potatoes
can also eliminate insect-pathogenic fungi, thus favoring
aphid outbreaks.
10
might be feasible to conserve them.
Other Resources
Insect Biocontrol Laboratory U.S. Department of
Agriculture: http://www.ba.ars.usda.gov/psi/ibl/
Biological Control Information Center, University of
North Carolina: http://cipm.ncsu.edu/ent/biocontrol/
Biological Control: A Guide to Natural Enemies
in North America: http://www.nysaes.cornell.edu/ent/
biocontrol/
Nematodes as Biological Control Agents of Insects:
http://nematode.unl.edu/wormepns.htm
The Association of Natural Biocontrol Producers:
http://www.anbp.org/
Photo Credits
Figs. 1,2,3,4,5 and 13: J.P. Michaud.
Figs.14 and 15: P.E. Sloderbeck.
Figs. 7 and 16: U. Wyss.
Fig. 8: Georgia Museum of Natural History.
Fig. 6: University of Idaho.
Fig. 12: E. Prado.
Figs. 9,10,11 and 17: unknown.
11
J.P. Michaud, P.E. Sloderbeck, and J.R. Nechols, entomologists
Kansas State University
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